Insect Herbivore Guild Interactions and Tall Thistle Population Dynamics: A Test of Conflicting Hypotheses

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Plants have a wide array of natural enemies, but the quantitative prediction of the consequences of interactions among these plant feeders (herbivores) remains difficult. Fundamental, unresolved questions include: (i) How do interactions among herbivores affect plant populations? (ii) Do interactions have synergistic or conflicting impacts? and, (iii) Does the outcome depend upon resources? There is an urgent need to improve our understanding of such interactions. The information is essential to our basic understanding of biological interactions in nature. Further, knowledge about such interactions has practical significance for advancing effective, environmentally safe biological control of weeds. This project examines the interaction of native insects with the native tall thistle (Cirsium altissimum) as a model system with four specific aims. These are to determine: (1) whether plant performance differs in relation to feeding by the main types of insect herbivores; (2) if herbivores have neutral, synergistic, or inhibitory effects on plant dynamics, (3) if the outcomes are related to resource availability; and (4) which of two current, conflicting hypotheses best explains the effects of multiple herbivores. These leading hypotheses are: first, that reductions in plant reproduction and density are highest when plants are exposed to a single, highly effective insect herbivore species; and, second, that these reductions are greatest when multiple insect herbivores attack different plant organs (e.g. roots, leaves, flowers). The four aims will be achieved by intensive measurement of insect feeding in natural C. altissimum populations along resource gradients, by insect exclusion experiments, and by development of an integral projection model. This research is significant because it addresses important, under-explored questions about ubiquitous biological interactions and because it has practical application in weed biocontrol. This research project also will expand educational opportunities for students from rural communities, an underrepresented group in higher education.